DVB represents the technological evolution that is going to replace the analog TeleVision (TV) broadcasting systems used for more than 50 years.
In particular, due to the enormous popularity gained by personal mobile communications, a promising evolution of DVB is the DVB-H (DVB-Handheld) system, by means of which TV will be made available to users of mobile communications terminals like mobile phones.
As known to those skilled in the art, the DVB-H system is an SFN (Single-Frequency Network) system based on OFDM (Orthogonal Frequency Division Multiplex). In an SFN, all transmitters in the network use the same channel/frequency. The OFDM is a modulation system in which the information is carried via a large number of individual (sub-)carriers, in a frequency multiplex scheme; each (sub-)carrier transports only a relatively small amount of information, and high data capacities are achieved using a large number of frequency-multiplexed carriers. Each carrier is modulated using QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) techniques, and has a fixed phase and amplitude for a certain time interval, referred to as the “symbol time”, during which a small portion of the information, called “symbol”, is carried. After that time period, the modulation is changed and the next symbol carries the next information portion. The symbol time is the inverse of the (sub-)carrier spacing, and this ensures orthogonality between the carriers.
Modulation and demodulation are accomplished using the IFFT (Inverse Fast Fourier Transform) and the FFT, respectively.
In order to demodulate the received signal, the generic receiver has to evaluate the symbol during the symbol time. This involves properly positioning an FFT evaluation time window, i.e., properly “synchronize” the time window for the OFDM demodulation of the received signals.
The paper of R. Brugger and D. Hemingway, “OFDM receivers—impact on coverage of inter-symbol interference and FFT window positioning”, EBU Technical Review, July 2003, pages 1-12, offers a general overview of the possible strategies for FFT window synchronization in OFDM receivers. These strategies are equally applicable to the T-DAB (Terrestrial-Digital Audio Broadcasting) and DBV-T (Digital Video Broadcasting-Terrestrial) systems.
In such systems, signals generally arrive at a generic receiver following different paths, corresponding to multiple transmitters and/or echoes of a same transmitted signal, to which there are associated different time delays; these different delays can cause ISI (Inter-Symbol Interference) at the receiver, because it is typically not possible to synchronize the FFT window to all the received signals: whichever the FFT window time positioning, there will always be some overlap with a preceding or following symbol in the transmission sequence. This ISI degrades the receiver's performance.
In order to allow, as much as possible, a constructive combination of the signals getting to the receiver through different paths, OFDM systems with multipath capabilities have been proposed, in which a “guard time interval” (sometimes also referred to as “guard space”) is provided for. The guard time interval consists in a cyclic prolongation of the useful symbol time of the signal; essentially, the normal symbol duration is extended, so that a complete symbol comprises, in addition to a useful part, a cyclic prolongation of every symbol, whose time duration corresponds to the guard interval. In the cited paper of R. Brugger and D. Hemingway, the prolongation is obtained by copying part of the symbol from the beginning of the symbol to the end, increasing the duration of the guard interval.
Thanks to the provision of the guard interval, the OFDM receiver can position in time the FFT window so that there is no overlap with a preceding or subsequent symbol, thus reducing to a minimum the ISI.
Before the actual deployment of the network in a geographic area of interest, a network planning is performed, exploiting specifically-designed software tools.
In the network planning phase, the geographic area of interest is usually subdivided into several relatively small elementary area elements, also referred to as pixels, for example squares of 50 m by 50 m. Based on an initial network configuration, with a certain positioning and radio equipment of the DVB-H transmission stations, the distribution of the electromagnetic field in every pixel is estimated, by means of an electro-magnetic field propagation simulator. The generic pixel is assumed to represent a virtual DVB-H receiver, i.e. it is assumed that, in the generic pixel, at least one DVB-H receiver is located. For each pixel, the signal-to-noise ratio (also referred to as the “C/I”, where C denotes the “useful” signal, and I denotes the interference) is estimated, to assess whether the network coverage in the considered pixel is adequate, or rather the network configuration should be modified to improve the network coverage.
In order to improve the reception quality, the received useful signal should be maximized, and the interference should be kept low; to this end, the network should be planned in such a way as to ensure that, in each pixel of the area of interest, most of the signals coming from different transmitters get to the considered pixel delayed from each other of less than the guard time interval. If this occurs, the received signal is not, or only scarcely, degraded by the ISI.